Ask a high-frequency power supply (RF generator) that feeds high-frequency power into a high-frequency load such as a plasma load (plasma source), a class-D high-frequency power supply device is known, for instance. Since the class-D high-frequency power supply device operates in a switch mode according to switching of high-frequency power amplifying elements, internal resistance Rin in the class-D high-frequency power supply device is determined by an ON-resistance value Ron in a saturated region of the high-frequency power amplifying elements. In general, the ON-resistance value Ron has resistance which is lower than characteristic impedance Z0 for transmitting output power.
The class-D high-frequency power supply device feeds the output power into the load device via a transmission path having the characteristic impedance Z0. Therefore, the impedance Zg0 viewed from the output end of the generator is designed in such a manner that it becomes equal to the characteristic impedance Z0 (Zg0=Z0) in a steady state, thereby maximizing the supplying of power.
The high-frequency power supply device outputs high-frequency waves generated in a high-frequency amplifier circuit internally provided, into the transmission path via an output circuit such as a power combining circuit and a matching circuit, and feeds the high-frequency waves into the load. In general, the impedance Zamp viewed from the high-frequency amplifier circuit is represented by the impedance that is obtained by impedance transformation of the impedance Zg0 at the output end of the high-frequency power supply device in a steady state, through the output circuit within the high-frequency power supply device.
FIG. 15 schematically illustrates a circuit of the class-D high-frequency power supply device. In FIG. 15A, the class-D high-frequency power supply device 101 allows the high-frequency amplifier circuit 112 to make direct current from the DC power source 111 higher in frequency, passes thus obtained high-frequency wave through the output circuit 113, and thereafter, feeds the high-frequency wave from the output end of the generator into the load 102, via the transmission path 104.
The high-frequency amplifier circuit 112 may include, for example, a bridge circuit 112a of the high-frequency power amplifying elements and a transformer 112b. The output circuit 113 may include, for example, a matching circuit 113a for matching impedance to the impedance Z0 of the transmission path 104, and a filter circuit 113b for removing a noise component. The impedance Zamp viewed from the high-frequency amplifier circuit 112 is obtained by impedance transformation of the impedance Zg0 at the output end of the class-D high-frequency power supply device 101 by using the impedance of the output circuit 113.
FIG. 15B briefly illustrates the impedance Zamp, showing a configuration that a circuit of AC voltage source Vin and internal resistance Rin substitutes for the DC power source 111 and the high-frequency amplifier circuit 112 including the bridge circuit 112a and the transformer 112b. The output power of this circuit is maximized when the relationship of Zamp=Rin=2RonN2 is established. However, in fact, there are restrictions by design of the high-frequency power amplifying elements and the DC power source part, and further, the impedance Zamp is required to be defined as a lagging load. Therefore, it is not necessarily defined as Zamp=Rin for maximizing power.
In the high-frequency power supply device, in order to prevent damage on the high-frequency source and unstable operations, caused by reflected waves due to an impedance mismatch on the transmission path, a configuration is suggested where the class-D high-frequency power supply device incorporates a 3-dB coupler to reduce the reflected waves by using an internal dummy load.
There is also known another configuration that prevents reflected waves from returning to the high-frequency source by placing a circulator on the transmission path, and converts the reflected waves into heat by a dummy load (see the part of “Background Art” of the Patent Document 1). In here, the circulator is a passive element having a function to output high-frequency signals inputted in a certain port among plural ports, to the next port only, preventing reflected waves from returning to the high-frequency source, whereby it is possible to avoid damage and unstable operations of the high-frequency source.
In the configuration where the 3-dB coupler is employed, however, a main body of the 3-dB coupler and the internal dummy load have to be implemented within the high-frequency power supply device, thus causing a problem that the configuration of the high-frequency power supply device may become large in size. Further in the configuration employing the 3-dB coupler, there is a problem that the required number of high-frequency amplifier circuits is a multiple of 2 of the number of the 3-dB couplers, and there is another problem that when a reflected wave is generated, reflected current passing through the high-frequency amplifier circuit may cause unbalance of over 200% at a maximum.
Therefore, in the configuration where the dummy load is employed, reflected waves are thermally converted by the dummy load that is connected to a port, causing a problem that energy usage efficiency is low. As a configuration for solving the problems above, there is suggested a power regeneration technique that extracts reflected harmonics from the transmission path, and converting the reflected harmonics being extracted into direct current, thereby regenerating high-frequency power (Patent Document 1).